1. Field of the Invention
The present invention relates to a radio communication device and, particularly, to a radio communication device with a time division duplex (TDD) scheme using the same carrier frequency for transmission and reception, such as a passive radio frequency identification (RFID) reader device.
2. Description of Related Art
In a passive RFID system, a radio tag operates by using a radio wave transmitted from a reader device as a power. For example, a passive radio tag generates an ASK-modulated reflected wave by changing impedance based on identification information unique to the radio tag when reflecting a non-modulated carrier transmitted from a reader device. The modulated reflected wave is called a backscattered signal. The reader device receives the backscattered signal as a response from the radio tag. In this manner, the passive RFID system adopts a time division duplex (TDD) scheme that uses the same carrier frequency for transmission and reception. In addition, the reader device in the passive RFID system continuously transmits a non-modulated carrier which is necessary for generation of a backscattered signal while the radio tag is transmitting a radio signal (a backscattered signal).
A frequency band available for radio communication and its maximum power are regulated by law or the like in each country. Further, suppression of a transmission power of a radio communication device contributes to reduction of power consumption. Therefore, a general radio communication device includes a circuit that measures a transmission power and a mechanism that makes control related to a transmission power based on a measurement result (cf. e.g. Japanese Unexamined Patent Application Publications Nos. 2003-332918, 2004-80290, 10-285058, 10-22757, 04-372227, 63-138807, and 61-163729). There are various ways of control related to a transmission power. Illustrative examples of control related to a transmission power includes (a) conforming that a transmission power is within an allowable range by a measured value of the transmission power, (b) adjusting the gain of a transmission signal in such a way that a transmission power becomes closer to a desired value, (c) stopping transmission output upon detecting the anomaly of a transmission power, and so on.